Filter system comprising a bulk acoustic wave resonator

ABSTRACT

The invention relates to a filter device equipped with at least one bulk acoustic wave resonator, which comprises a resonator unit and a reflection element ( 2 ) comprising several layers ( 6, 7, 8 ). The layers ( 6, 7, 8 ) each comprise a mixture of at least two materials. Within each layer ( 6, 7, 8 ), the composition of the mixture varies continuously and periodically relative to the layer thickness.

The invention relates to a filter device equipped with at least one bulkacoustic wave resonator, which comprises a resonator unit and areflection element. The invention also relates to a wireless datatransmission system, a transmitter, a receiver and a mobile radio deviceequipped with a filter arrangement. The invention also relates to abulk-wave resonator.

The very rapid developments in the area of mobile radio and the constantminiaturization of cordless telephones are leading to more stringentrequirements as regards the individual components. A high degree ofselectivity is necessary in the high-frequency section in order toprotect the receiver from the increasing number of possibly interferingsignals from other systems. This is achieved, for example, by bandpassfilters, which allow only a limited frequency band through and suppressall frequencies above and below this range.

Such a filter may be, for example, a bulk acoustic wave filtercomprising bulk acoustic wave resonators which are also designated BulkAcoustic Wave (BAW) filters. Bulk acoustic wave resonators consist inprinciple of three components. The first component generates theacoustic wave and comprises a piezoelectric layer. Two electrodes, whichare affixed above and below the piezoelectric layer, represent thesecond component. The third component, i.e. the reflection element, hasthe task of isolating the substrate acoustically from the oscillationsgenerated by the piezoelectric layer.

One option for the acoustic isolation of the substrate from theoscillations of the piezoelectric layer is formed by acousticinterference filters, which comprise several λ/4 layers of materialswith a high and low acoustic impedance. A bulk acoustic wave resonatorof this kind is known from, for example, U.S. Pat. No. 5,646,583.

Particularly in the passband of a filter arrangement comprising bulkacoustic wave resonators, suitable acoustic decoupling of the bulkacoustic wave resonators from the substrate is necessary in order toachieve a good quality of the bulk acoustic wave resonators.

It is therefore an object of the invention to provide a filter devicewith at least one bulk acoustic wave resonator, which, particularly inthe passband of the filter arrangement, exhibits efficient acousticisolation of the substrate from the oscillations of the piezoelectriclayer.

This object is achieved by a filter device equipped with at least onebulk acoustic wave resonator, which comprises a resonator unit and areflection element, wherein reflection element comprises several layers,which each comprise a mixture of at least two materials, wherein, withineach layer, the composition of the mixture varies continuously andperiodically relative to the layer thickness.

Filter devices of this kind exhibit in the passband of the bulk acousticwave filter device a suitable acoustic decoupling of the bulk acousticwave resonators from the substrate. In addition, outside the passband,the reflection element exhibits a high transmission for acoustic waves,so the bulk acoustic wave filter device is suitably coupled to thesubstrate in this area. As a result, higher harmonic frequencies outsidethe passband are very effectively suppressed and the filter arrangementexhibits no undesired resonances here. Overall, a filter device of thiskind is equipped with a clearly defined stop band.

It is preferred if the reflection element comprises a mixture of SiO₂and Ta₂O₅ or SiO₂ and Si₃N₄.

These materials and mixtures of these materials all have good electricalinsulation properties, and can easily be produced and applied to asubstrate.

The invention also relates to a transmitter, a receiver, a mobile radiodevice and a wireless data transmission system equipped with a filterdevice with at least one bulk acoustic wave resonator, which comprises aresonator unit and a reflection element, wherein the reflection elementcomprises multiple layers, which each comprise a mixture of at least twomaterials, wherein, within each layer, the composition of the mixturevaries continuously and periodically relative to the layer thickness.The invention also relates to a bulk acoustic wave resonator, whichcomprises a resonator unit and a reflection element, wherein thereflection element comprises multiple layers, which each comprise amixture of at least two materials, wherein, within each layer, thecomposition of the mixture varies continuously and periodically relativeto the layer thickness.

The invention will be described in detail hereinafter with reference tofive Figures and three embodiments, to which, however, the invention isnot restricted.

FIG. 1 is a cross-sectional representation of the structure of a filterarrangement of bulk acoustic wave resonators.

FIG. 2 shows the periodic variation of the composition of a mixturecomprising at least two materials in the reflection element of a bulkacoustic wave filter device.

FIG. 3 shows the reflection behavior of a reflection element comprisinginterference layers.

FIG. 4 and FIG. 5 show the reflection behavior of reflection elements inaccordance with the invention.

According to FIG. 1, a bulk acoustic wave filter device comprises asubstrate 1 which consists for example of a ceramic material, a ceramicmaterial with a planarization layer of glass, a glass-ceramic material,a glass material, a semiconductor material, such as for example silicon,GaAs, InP, SiC or GaN, or sapphire. If a semiconductor material is usedas the substrate 1, a passivation layer of SiO₂ or glass, for example,may also be applied. A reflection element 2 is located on the substrate1. The reflection element 2 comprises a plurality of layers 6, 7, 8. Thetotal number of layers of reflection element 2 is preferably between 3and 12. The layers 6, 7, 8 of the reflection element 2 comprise amixture of at least two materials. The layers 6, 7, 8 preferablycomprise a mixture of SiO₂ and Ta₂O₅ or SiO₂ and Si₃N₄. It isadvantageous if the mixture is an amorphous mixture. The compositionwithin a layer varies continuously and periodically, for instancesinusoidally, with layer thickness d as a period, relative to thedirection of layer thickness d, as shown in FIG. 2. The layer thicknessd of each layer is derived from the formula:$d = \frac{( {v_{Material1} + v_{material2}} )}{4f_{c}}$

-   -   wherein ν_(Material1) is the acoustic velocity of the first        material of the mixture, ν_(Material2) is the acoustic velocity        of the second material of the mixture, and f_(c) is the        operating frequency of the bulk acoustic wave filter device at        which the reflection of the reflection element 2 should be 100%.

On the reflection element 2 are positioned resonator units, which eachcomprise a first electrode 3, a piezoelectric layer 4 and a secondelectrode 5.

The electrodes 3 and 5 are preferably made of a suitably conductivematerial with low acoustic absorption.

Examples of materials that may be used as the material for thepiezoelectric layer 4 are AlN, ZnO, Pb(Zn_(1/3)Nb_(2/3))O₃—PbTiO₃,Pb(Mg_(1/3)Nb_(2/3))O₃-PbTiO₃, Pb(Ni_(1/3)Nb_(2/3))O₃—PbTiO₃,Pb(Sc_(1/3)Nb_(2/3))O₃—PbTiO₃,Pb(Zn_(1/3)Nb_(2/3))_(1-x-y)(Mn_(1/2)Nb_(1/2))_(x)Ti_(y)O₃ (0≦x≦1,0≦y≦1), Pb(In_(1/2)Nb_(1/2))O₃—PbTiO₃, Sr₃TaGa₃Si₂O₁₄,K(Sr_(1-x)Ba_(x))₂Nb₅O₁₅ (0≦x≦1), Na(Sr_(1-x)Ba_(x))₂Nb₅O₁₅ (0≦x≦1),BaTiO₃, (K_(1-x)Na_(x))NbO₃ (0≦x≦1), KTaO₃, (Bi,Na,K,Pb,Ba)TiO₃,(Bi,Na)TiO₃, Bi₇Ti₄NbO₂₁, (K_(1-x)Na_(x))NbO₃—(Bi,Na,K,Pb,Ba)TiO₃(0≦x≦1), a(Bi_(x)Na_(1-x))TiO_(3-b)(KNbO_(3-c))½(Bi₂O₃—Sc₂O₃) (0≦x≦1,a+b+c=1), (Ba_(a)Sr_(b)Ca_(c))Ti_(x)Zr_(1-x)O₃ (0≦x≦1, a+b+c=1),(Ba_(a)Sr_(b)La_(c))Bi₄Ti₄O₁₅ (a+b+c=1), Bi₄Ti₃O₁₂, LiNbO₃,La₃Ga_(5.5)Nb_(0.5)O₁₄, La₃Ga₅SiO₁₄, La₃Ga_(5.5)Ta_(0.5)O₁₄ andPbZr_(x)Ti_(1-x)O₃ (0≦x≦1) with and without dopants of La, Mn, Fe, Sb,Sr, Ni or combinations of these dopants.

A protective layer of an organic or an inorganic material or acombination of these materials may be applied above the filter device.Examples of organic materials that may be used are polybenzocyclobuteneor polyimide, and examples of inorganic materials are Si₃N₄, SiO₂ orSi_(x)O_(y)N_(z) (0≦x≦1, 0≦y≦1, 0≦z≦1). Alternatively, a thin layer ofSiO₂ may be applied to one or more bulk acoustic wave resonators of thefilter device for specific detuning of the bulk acoustic resonator. Itmay be preferable for the thin layer of SiO₂ to be applied only to thesecond electrode 5 of a bulk acoustic wave resonator. The layerthickness of the thin layer of SiO₂ preferably amounts to between 10 and100 nm.

Production of a reflection element 2, which comprises layers 6, 7, 8made of a mixture whose composition varies continuously andperiodically, may take place by, for instance, plasma-enhanced chemicalvapor deposition (PECVD).

For production of layers 6, 7, 8 comprising SiO₂ and Ta₂O₅, the volumeof fed-in gases, for instance Si(OC₂H₅)₄, Ta(OC₂H₅)₅ and O₂, is variedcontinuously periodically. For production of layers 6, 7, 8 comprisingSiO₂ and Si₃N₄, the volume of fed-in gases, for instance SiH₄, N₂O andNH₃, is varied continuously periodically.

A filter device in accordance with the invention may be used, forinstance, for signal filtering in a wireless data transmission system,in a mobile radio device, in a transmitter or in a receiver.

Embodiments of the invention representing examples of implementation aredescribed below.

Embodiment 1

COMPARISON EXAMPLE

A filter device made of bulk acoustic wave resonators with a reflectionelement 2 made of interference layers is applied to a substrate 1 madeof silicon. To this end, a reflection element 2, which comprises sevenlayers of alternating SiO₂ and Ta₂O₅, is applied to the substrate 1. Thefirst layer 6 on the substrate 1 comprises SiO₂. The layer thickness ofthe layers with SiO₂ was 524 nm, and the layer thickness of the layerswith Ta₂O₅ was 409 nm. On the topmost layer 8 of reflection element 2,which comprises SiO₂, are located the individual bulk acoustic waveresonators, which each comprise a first electrode 3, a piezoelectriclayer 4 and a second electrode 5. The first electrode 3 comprises Alwith a layer thickness of 300 nm. A 1381 nm thick layer of AlN isapplied to each first electrode 3 as the piezoelectric layer 4. On eachpiezoelectric layer 4 is located a 150 nm thick second electrode 5 madeof Al. The individual bulk acoustic wave resonators are electricallyconnected on substrate 1 in such a way that a filter device for thefrequency 2.85 GHz was obtained.

FIG. 3 shows the reflection behavior of the reflection element 2 of thisfilter arrangement.

Embodiment 2

A filter device made of bulk acoustic wave resonators with a reflectionelement 2 is applied to a substrate 1 made of silicon. To this end, areflection element 2, which comprises ten layers, is applied to thesubstrate 1. Each layer comprises a mixture of SiO₂ and Ta₂O₅; thequantity of each component in a layer 6, 7, 8 varies sinusoidally withthe layer thickness d of a layer 6, 7, 8 constituting the period. FIG. 2shows the composition within the layers 6, 7, 8 relative to the layerthickness. Curve 9 corresponds to Ta₂O₅, and curve 10 to SiO₂. The layerthickness d of each layer 6, 7, 8 was 466 nm. On the topmost layer 8 ofreflector element 2 are located the individual bulk acoustic waveresonators, which each comprise a first electrode 3, a piezoelectriclayer 4, and a second electrode 5. The first electrode 3 comprises Alwith a layer thickness of 200 nm. A 1679 nm thick layer of AlN isapplied to each first electrode 3 as the piezoelectric layer 4. On eachpiezoelectric layer 4 is located a 200 m thick second electrode 5 madeof Al. The individual bulk acoustic wave resonators are electricallyconnected on substrate 1 in such a way that a filter device for thefrequency 2.85 GHz was obtained.

FIG. 4 shows the reflection behavior of the reflection element 2 of thisfilter device. A filter device of this kind was used for signalfiltering in the high-frequency section of a mobile radio device.

Embodiment 3

A filter device made of bulk acoustic wave resonators with a reflectionelement 2 is applied to a substrate 1 made of silicon. To this end, areflection element 2, which comprises ten layers, is applied to thesubstrate 1. Each layer comprises a mixture of SiO₂ and Si₃N₄; thequantity of each component in a layer 6, 7, 8 varies sinusoidally withthe layer thickness d constituting the period. The layer thickness d ofeach layer 6, 7, 8 was 744 nm. On the topmost layer 8 of reflectorelement 2 are located the individual bulk acoustic wave resonators,which each comprise a first electrode 3, a piezoelectric layer 4, and asecond electrode 5. The first electrode 3 comprises Al with a layerthickness of 200 nm. A 1650 nm thick layer of AlN is applied to eachfirst electrode 3 as the piezoelectric layer 4. On each piezoelectriclayer 4 is located a 200 nm thick second electrode 5 made of Al. Theindividual bulk acoustic wave resonators are electrically connected onthe substrate 1 in such a way that a filter device for the frequency2.85 GHz was obtained.

FIG. 5 shows the reflection behavior of reflection element 2 of thisfilter device. A filter device of this kind was used for signalfiltering in the high frequency section of a mobile radio device.

1. A filter device equipped with at least one bulk acoustic waveresonator, which comprises a resonator unit and a reflection element(2), wherein the reflection element (2) comprises several layers (6, 7,8), which each comprise a mixture of at least two materials, wherein,within each layer (6, 7, 8), the composition of the mixture variescontinuously and periodically relative to the layer thickness.
 2. Afilter device as claimed in claim 1, characterized in that thereflection element (2) comprises a mixture of SiO₂ and Ta₂O₅ or SiO₂ andSi₃N₄.
 3. A mobile radio device equipped with a filter device with atleast one bulk acoustic wave resonator, which comprises a resonator unitand a reflection element (2), wherein the reflection element (2)comprises several layers (6, 7, 8), which each comprise a mixture of atleast two materials, wherein, within each layer (6, 7, 8), thecomposition of the mixture varies continuously and periodically relativeto the layer thickness.
 4. A transmitter equipped with a filter devicewith at least one bulk acoustic wave resonator, which comprises aresonator unit and a reflection element (2), wherein the reflectorelement (2) comprises several layers (6, 7, 8), which each comprise amixture of at least two materials, wherein, within each layer (6, 7, 8),the composition of the mixture varies continuously and periodicallyrelative to the layer thickness.
 5. A receiver equipped with a filterdevice with at least one bulk acoustic wave resonator, which comprises aresonator unit and a reflection element (2), wherein the reflectionelement (2) comprises several layers (6, 7, 8), which each comprise amixture of at least two materials, wherein, within each layer (6, 7, 8),the composition of the mixture varies continuously and periodicallyrelative to the layer thickness.
 6. A wireless data transmission systemequipped with a filter device with at least one bulk acoustic waveresonator, which comprises a resonator unit and a reflection element(2), wherein the reflector element (2) comprises multiple layers (6, 7,8), which each comprise a mixture of at least two materials, wherein,within each layer (6, 7, 8), the composition of the mixture variescontinuously and periodically relative to the layer thickness.
 7. A bulkacoustic wave resonator, which comprises a resonator unit and areflection element (2), wherein the reflection element (2) comprisesseveral layers (6, 7, 8), which each comprise a mixture of at least twomaterials, wherein, within each layer (6, 7, 8), the composition of themixture varies continuously and periodically relative to the layerthickness.